1. Technical Field
This disclosure relates to a driving circuit having a switching circuit for driving a load, particularly for driving a load with high inrush current such as a bulb.
2. Description of the Related Art
One approach that has been used for a driving circuit having a switching circuit connected to a load is shown in FIG. 1. The driving circuit 1 according to FIG. 1 has a switching circuit 2 which is coupled to a load 3, which is in the present example a bulb or incandescent lamp. The switching circuit 2 may comprise one or more switching elements such as transistors and has a control input for switching the switching circuit 2 in a conducting or non-conducting state, and has a controlled path coupled to the load 3 at a first node and coupled to a reference potential GND (in the present example ground potential) at the other node. At the terminal opposite to the switching circuit 2, the bulb 3 is connected to a supply voltage V.
In order to operate the bulb 3, the switching circuit 2 is operated to switch into a conducting state, so that the bulb 3 is connected between the supply voltage V and the reference potential GND. FIG. 2 shows a signal diagram which depicts an example of an inrush current of a driving circuit of the principle as shown in FIG. 1 when driving a bulb (e.g. a bulb with nominal power of 5 W). As can be seen from the signal diagram according to FIG. 2, when switching the switching circuit 2 from non-conducting state to conducting state at time 0, the inrush current of the bulb 3 is, in this example, approximately 10 times higher than the average or stationary current. In the present example, the high inrush current results from heating up the glow filament of the bulb. Usually, such high inrush current is disadvantageous with respect to the switching circuit 2, since the switching circuit 2 must be capable to switch a high inrush current which is a multiple of the stationary current, which may result in high thermal stress, and big chip size as the switching circuit has to have a large size for switching high inrush currents and dissipating the power loss.
In order to avoid such critical inrush currents, another common prior approach is to limit the inrush current at a fixed value, for example limit the inrush current at a fixed value of 0.8 A in the example of FIG. 3. In this regard, FIG. 3 shows a signal diagram depicting another example of an inrush current of a driving circuit of the principle as shown in FIG. 1 with limitation of the inrush current at a fixed value (here 0.8 A) according to this common approach. However, this approach also has several disadvantages, such as high power dissipation on the integrated circuit with the switch (which results in high power dissipation on silicon), high thermal stress, big chip size, and thus increased manufacturing costs of the integrated circuit. Particularly, when limiting the inrush current at a fixed value, a considerable portion of the potential between the supply voltage V and the reference potential GND (which is a relative large voltage drop) must occur across the switching circuit 2 as a result of the rather low inrush current and the rather low voltage drop across the bulb 3 during the heating up period.
In FIG. 4, there is shown an exemplary driving circuit for driving a load with high inrush current, such as a bulb, according to another prior approach using a pulse width modulated control circuit for driving the load. The driving circuit according to FIG. 4 employs a so called “soft start function” or “over-current recovery mode”. Like in the example of FIG. 1, the driving circuit 10 according to FIG. 4 comprises a switching circuit 12 which is coupled with a controlled path between a bulb 3 and a reference potential GND, the bulb 3 connected at its other end to supply voltage V. The switching circuit 12 has a control input for switching the switching circuit 12 into a conducting or non-conducting state, wherein in the present example the control input of the switching circuit 12 is coupled to a gate driver 13. The gate driver 13 is coupled to a signal line 17 for receiving an enable signal for activating the driving circuit 10 (signal “ON”) or disabling the driving circuit 10 (signal “OFF”). The driving circuit 10 further comprises a detecting circuit 14 for detecting an over current in the controlled path of the switching circuit 12. Particularly, the detecting circuit 14 detects if the current in the controlled path of the switching circuit 12 between the bulb 3 and the reference potential GND equals or is higher than a particular threshold value. An output of the detecting circuit 14 is coupled to a latch 15, an output of which is coupled to the gate driver 13 and to a recovery timer 16. A reset input of the latch 15 is coupled to an output of the recovery timer 16.
The function of the driving circuit according to FIG. 4 will now be explained with reference to FIG. 5. FIG. 5 shows a signal diagram depicting an example of a limited inrush current of a driving circuit of the principle as shown in FIG. 4 when driving a bulb. The signal diagram of FIG. 5 also shows in comparison to the limited inrush current an unlimited inrush current as shown and discussed above with reference to FIG. 2. In the signal diagram of FIG. 5, when switching the switching circuit 12 to a conducting state, the load current of the bulb 3 increases until the current reaches a threshold value IT which is detected by detecting circuit 14. In other words, the detecting circuit 14 detects an over current in the controlled path of switching circuit 12 and produces an output signal which is latched in the latch 15. The latched over current output signal is provided to the gate driver 13 which operates the switching circuit 12 to switch to the non-conducting state. As a consequence, the load current of the bulb 3 decreases as shown in FIG. 5. At the same time, the latched over current output signal of the detecting circuit 14 is also provided to the recovery timer 16 which starts to count. The recovery timer 16 produces a respective output signal after a particular time period from starting counting has elapsed. This output signal from the recovery timer is supplied to the reset input of the latch 15 which, when reset, causes the gate driver 13 to switch the switching circuit 12 again in conducting state. As a result, the load current of the bulb 3 again increases until it reaches the threshold value IT detected by the detecting circuit 14. This process as described above is repeated until the load current keeps below the threshold value IT and develops towards the stationary load current as shown in FIG. 5.
The pulse width modulated load current of the principle as shown in FIG. 5 provides sufficient average current to power up the load (in the present example, to heat up the bulb) until the load reaches stationary operating condition. Advantages with respect to limiting the inrush current as shown in FIG. 3 result from the fact that the power for heating up the bulb is determined fromP=I2Rload 
with P being the power, I being the load current through the bulb and Rload being the resistance of the bulb. Consequently, advantages as compared to the approach according to FIG. 3 are lower power dissipation on the integrated circuit (lower power dissipation on silicon), lower thermal stress, lower chip size and, thus, cost reduction in manufacturing costs, and a higher lifetime of the bulb.
When having a driving circuit 10 as shown with reference to FIG. 4 for driving a load 3, in some applications it may be necessary to drive loads (particularly bulbs) with higher power, but using the driving circuit 10 of the principle as shown in FIG. 4 which is designed for driving loads with lower power. In other words, it is not desirable to provide different driving circuits 10, with keeping one driving circuit design for driving loads with lower power and another driving circuit design for driving loads with higher power.
A solution for driving loads with higher power, but using a driving circuit design which is capable of driving loads with lower power, is to use two switching circuits connected in parallel to a load with higher power. For example, when using a driving circuit design as shown in FIG. 4, another driving circuit 10 may be coupled to a bulb 3 with higher power having a second switching circuit which is coupled to the bulb 3 in parallel to the switching circuit 12 as shown in FIG. 4. As a consequence of using two switching circuits in parallel, higher load currents may be switched in order to drive loads with higher power. However, a problem may arise in the event that the two switching circuits are operated to switch to a conducting state at different times, such as shown in FIG. 6. In FIG. 6, a signal diagram is shown depicting an example of a limited inrush current of the principle as shown in FIG. 5 when driving a bulb of higher power and using two driving circuits of the principle as shown in FIG. 4 connected to the bulb in parallel. As shown in FIG. 6, the switching circuit of one of the driving circuits (“Switch 1”) is switched to a conducting state at a time which is different from the time of when the switching circuit of the other driving circuit (“Switch 2”) is switched to the conducting state. As a result of the different switching times, it is not possible to get the needed higher load current for driving the load with higher power since the two pulse width modulated currents of the two driving circuits will not provide sufficient average current to power up the load (e.g. heat up the bulb according to P=I2Rload). Consequently, this approach would not work properly when driving bulbs for the reasons as set out above.
Therefore, it would be beneficial to provide a driving circuit which is capable of driving loads, such as bulbs, with higher power.